This invention relates to enhanced phosphate removal in an activated sludge sewage or wastewater (used interchangeably herein) treatment process.
Sidestream biological phosphorus removal is any process in which a portion or all of the return activated sludge is separated or concentrated from a phosphate-free, or phosphate-reduced effluent (in a secondary settling zone) with the separated or concentrated sludge maintained anaerobically to induce the organisms contained therein to release internal phosphate to provide a relatively (compared to the influent) concentrated phosphate supernatant. The anaerobic sludge organism mass is then returned to the influent or primary wastewater stream or directly to the aeration basin for mixing with influent wastewater under aerobic conditions which induce the organisms to take up most or all of the influent and process-released phosphate accompanying the return anaerobic sludge. The aeration basin discharges to a settling basin or clarifier and, after settling or separation, the mixed liquor thus treated leaves the supernatant effluent phosphate-free, or nearly so, for discharge into receiving waters or for further treatment. The process can thus avail itself of two "sinks" for ultimate removal of phosphorus from the wastewater: (1) chemical removal by precipitation of the anaerobically released phosphate contained in the supernatant of the anaerobic zone, and (2) removal of phosphate-rich activated sludge through normal sludge wasting. For many applications, the present invention will achieve the desired results using only the second of these sinks, normal sludge wasting.
The sidestream processes, which are described and claimed in U.S. Pat. Nos. 3,236,766; 3,654,146; 3,654,147; 3,681,235; 3,730,882; 3,756,946; 4,042,493; 4,141,822; 4,175,041; and 4,183,808, the disclosures of which are incorporated herein by reference, have gained widespread acceptance as an efficacious means for removing phosphorus from municipal wastewater economically to produce final effluents containing only 1.0 mg/L, or less, total phosphorus. Previous methods relied upon the more costly and more sludge productive chemical precipitation of phosphate from the entire wastewater flow.
The active agent in all biological phosphorus removal processes is the microorganism constituent of the activated sludge. It is these sludge organisms which must be cycled through aerobic and anaerobic treatment to induce the phosphate uptake and release activities essential for the processes. Applying anaerobic treatment to the activated sludge only, in whole or in part, constitutes a major advantage for the sidestream processes over the fullstream processes since less tank volume is required to retain the microorganisms under anaerobic conditions in the sidestream processes than to retain the same mass of microorganisms under anaerobic conditions for the same time period in the fullstream processes.
Siebritz et al, "A Parametric Model for Biological Excess Phosphorus Removal", presented at the I. A. W. P. R. Post Conference Seminar on Phosphate Removal, Pretoria, South Africa (1982) and Ekama et al, "Considerations in the Process Design of Nutrient Removal Activated Sludge Processes", Department of Civil Engineering, University of Capetown, Rondebosch, 7700, South Africa (1982), report on rates of release of phosphate from anaerobic microorganisms. They report that when additional dissolved organic matter is added to anaerobic wastewater microorganisms in fullstream processes, substantial quantities of phosphate are released by the organisms more rapidly than when no such organic matter is added. The added organic matter can be in the form of BOD/COD-containing wastewater.
U.S. Pat. No. 4,042,493 to Matsch et al discloses a sidestream phosphate removal process in which primary effluent is used to elutriate the phosphate secreted in the bottom of the phosphate stripper tank. The phosphate is thus conveyed to the supernatant level and flows over the effluent launder to the stripper tank. In this process, the anaerobic retention period applied to the sludge in the stripper tank is typically 10-16 hours. The primary effluent is introduced into the bottom of the stripper to effect a counter-current elutriation. Hence, initial contact of the BOD/COD-containing primary effluent is with the portion of the stripper tank sludge at the bottom of the tank. This sludge has already been held under anaerobic conditions for the stated 10-16 hour retention period. Even if the sludge content of the stripper tank were uniformly mixed, the average retention time of the organisms contacting the organic matter introduced with the primary effluent would already be half of the anaerobic retention period. This constitutes a period during which much of the releasable phosphate would have been secreted by the sludge organisms. In addition, the primary effluent is rapidly dispersed throughout the tank volume, quickly diluting the effective BOD/COD level to which the sludge organisms are exposed. Because of these factors, no significant additional phosphate is released when these organisms are contacted by the primary effluent. Thus, the effect of adding BOD/COD-containing wastewater on the ability of the organisms to release phosphate rapidly is small, or absent, in the process of Matsch et al.
S-E. Lee et al, "The Effects of Aeration Basin Configuration on Activated Sludge Bulking at Low Organic Loading", Water Science and Technology, 14, 407-427, Capetown, 1982, report on a variation on the activated sludge process in which return activated sludge is mixed with primary effluent in a small compartment, called a "cell selector", just before being passed to the aeration basin. The purpose of this procedure is to minimize the presence of foam-producing organisms which are a frequent problem in activated sludge plants. It accomplishes this by imposing a high initial BOD/COD-to-microorganism ratio by introducing primary effluent to the microorganisms while the former is still untreated. The Lee et al innovation has no direct bearing upon the phosphorus removal process, but provides for increasing the effective concentration of BOD/COD "seen" by the microorganisms in the activated sludge process.